Database 使用updatejoin触发器

我不明白错误是从哪里来的。。。 我的代码中显示以下错误。我已尝试搜索，但未找到 任何有用的东西

从命令中的第31行开始时出错- 从项目中删除，其中项目ID=104

错误报告- ORA-04091:表DB20168002。项_必要条件正在变异，触发器/函数可能看不到它

ORA-06512：位于“DB20168002.TR”第2行 ORA-04088:执行触发器“DB20168002.TR”时出错

drop table Item_Master;
drop table Item_Requisite;
create table Item_Master(Item_ID number primary key,Description varchar2(50),Bal_Stock number);
insert into Item_Master values(101,'keyboard',80);
insert into Item_Master values(102,'lathe_machine',70);
insert into Item_Master values(103,'compass',100);
insert into Item_Master values(104,'compass',90);
insert into Item_Master values(105,'Ammeter',60);

create table Item_Requisite(Item_ID number primary key,Dept_Code varchar2(50),Quantity number);
insert into Item_Requisite values(101,'cs',30);
insert into Item_Requisite values(102,'mech',20);
insert into Item_Requisite values(103,'civil',25);
insert into Item_Requisite values(104,'chem',35);
insert into Item_Requisite values(105,'elect',80);

create or replace trigger tr
before delete 
on
Item_Requisite
for each row
begin
    MERGE INTO Item_Master t1
    USING Item_Requisite t2
    ON (t1.Item_ID=t2.Item_ID)
    WHEN MATCHED THEN 
    UPDATE SET t1.Bal_Stock=t1.Bal_Stock-t2.Quantity
    where t1.Item_ID=104;   
end;
/
delete from Item_Requisite where Item_ID=104;
/
select * from Item_Master;

---

不要

合并
；您没有执行任何
INSERT
（而
MERGE
被称为upsert（在单个操作中更新/插入）。
update
将执行以下操作：

SQL> create or replace trigger tr
  2  before delete
  3  on
  4  Item_Requisite
  5  for each row
  6  begin
  7    update item_master t1 set
  8      t1.bal_stock = t1.bal_stock - :old.quantity
  9      where t1.item_id = 104
 10        and :old.item_id = t1.item_id;
 11
 12  --    MERGE INTO Item_Master t1
 13  --    USING Item_Requisite t2
 14  --    ON (t1.Item_ID=t2.Item_ID)
 15  --    WHEN MATCHED THEN
 16  --    UPDATE SET t1.Bal_Stock=t1.Bal_Stock-t2.Quantity
 17  --    where t1.Item_ID=104;
 18  end;
 19  /

Trigger created.

SQL> delete from Item_Requisite where Item_ID=104;

1 row deleted.

SQL> select * from Item_Master;

   ITEM_ID DESCRIPTION                                         BAL_STOCK
---------- -------------------------------------------------- ----------
       101 keyboard                                                   80
       102 lathe_machine                                              70
       103 compass                                                   100
       104 compass                                                    55
       105 Ammeter                                                    60

SQL>

不要
MERGE
；您没有执行任何
INSERT
（而
MERGE
被称为upsert（在单个操作中更新/插入）。
update
将执行以下操作：

SQL> create or replace trigger tr
  2  before delete
  3  on
  4  Item_Requisite
  5  for each row
  6  begin
  7    update item_master t1 set
  8      t1.bal_stock = t1.bal_stock - :old.quantity
  9      where t1.item_id = 104
 10        and :old.item_id = t1.item_id;
 11
 12  --    MERGE INTO Item_Master t1
 13  --    USING Item_Requisite t2
 14  --    ON (t1.Item_ID=t2.Item_ID)
 15  --    WHEN MATCHED THEN
 16  --    UPDATE SET t1.Bal_Stock=t1.Bal_Stock-t2.Quantity
 17  --    where t1.Item_ID=104;
 18  end;
 19  /

Trigger created.

SQL> delete from Item_Requisite where Item_ID=104;

1 row deleted.

SQL> select * from Item_Master;

   ITEM_ID DESCRIPTION                                         BAL_STOCK
---------- -------------------------------------------------- ----------
       101 keyboard                                                   80
       102 lathe_machine                                              70
       103 compass                                                   100
       104 compass                                                    55
       105 Ammeter                                                    60

SQL>

您正在尝试读取和更新创建触发器的同一个表，因此会出现错误。使用复合触发器可避免此错误您正在尝试读取和更新创建触发器的同一个表，因此会出现错误。使用复合触发器可避免此错误